The metabolic syndrome (MetS) is a constellation of biochemical and physical derangements that afflicts nearly 20-25% of individuals worldwide. It is increasingly appreciated that chronic inflammation is a unifying underlying mechanism not only for the development of MetS but also for the subsequent development of atherothrombotic cardiovascular disease - the primary source of morbidity and mortality in these individuals. Recent studies highlight an essential role for cells of the monocyte/macrophage lineage in the development of MetS and its consequences such as atherothrombosis. Cells of the monocyte/macrophages lineage exhibit remarkable plasticity that allows them to modulate their phenotype and efficiently respond to environmental signals and change their phenotype. For purposes of simplicity, a model system that classifies inflammatory macrophages as M1 and anti-inflammatory macrophages as M2 macrophages has been developed. The balance of these two subsets is thought to critically influence the physiologic and pathologic inflammatory response yet the molecular determinants of their speciation and function remain poorly understood. Kruppel-like factors (KLFs) are zinc-finger transcription factors implicated in a wide spectrum of biological processes including hematopoietic biology. Although previous studies by our group and others implicated KLF4 in myeloid cell biology, the in vivo physiological relevance has not been elucidated. Based on our preliminary studies, KLF4 expression is identified with the M2 population and strongly reduced in M1 macrophages - observations that are recapitulated in human inflammatory paradigms in vivo. Gain and loss-of-function studies reveal that KLF4 promotes an M2 genetic program and inhibits M1 target genes. Mice bearing myeloid-specific deletion of KLF4 exhibit a characteristic M1 phenotype as evidenced by enhanced bactericidal activity. Further, in response to a high fat diet (HFD), these animals develop numerous features consistent with MetS including obesity, dyslipidemia, insulin resistance, and a pro-atherogenic/pro-thrombotic state. These observations provide the foundation for the central hypothesis that KLF4 is an upstream molecular switch governing macrophage subset specification and function. To better understand the precise role of KLF4 in macrophage polarization and function, three aims are proposed. In Aim 1, we will delineate the upstream mechanisms governing KLF4 expression in myeloid cells. In Aim 2, we will determine the molecular basis for KLF4's ability to regulate the M1/M2 phenotype. In Aim 3, studies will determine the effect of altered myeloid KLF4 expression on insulin resistance and atherothrombosis. Collectively, these studies will elucidate the molecular basis for KLF4-mediated polarization towards the M2 phenotype and the functional consequences of KLF4 sufficiency and deficiency on insulin resistance and atherothrombosis.